The present subject matter relates generally to methods for the removal of sulfur compounds from hydrocarbon streams. More specifically, the present subject matter relates to the methods for the removal of carbon disulfide from petrochemical grade naphtha fed to an ethylene plant, by use of selective adsorbents.
Hydrocarbons used by industry should be produced to be as pure as necessary without the presence of contaminants over specified limits. Conventionally, many of the organic sulfur compounds fed to an ethylene cracker decompose to form hydrogen sulfide. While some thermal decomposition of carbon disulfide occurs, the remaining carbon disulfide in the cracker remains unconverted. The petrochemical grade naphtha fed to the cracking unit has to meet the product specification limits of carbon disulfide for any further use. Therefore, there is a need to remove carbon disulfide contaminants from the petrochemical naphtha to meet the product specifications for commercial use.
The presence of carbon disulfide in the cracked stream also affects downstream processing. Carbon disulfide can poison the selective hydrogenation catalysts that are generally used to remove acetylenes and dienes in olefin plants. The noble metal catalysts are very sensitive to sulfur and degrade in the presence of sulfur contaminants. Carbon disulfide also contaminates C4 and C5 olefins and dienes production in polymer plants and can be corrosive when it undergoes transformation to H2S. Therefore, there is a need for a process for removal of carbon disulfide from the hydrocarbon streams.
In one prior art process, the cracked gas is directed to acid gas scrubber for caustic wash. The acidic contaminants and hydrogen sulfide are readily removed by caustic wash. However, carbon disulfide contaminants present in the gas are not effectively removed by caustic wash.
Carbon disulfide (CS2) is a contaminant found in refinery and condensate hydrocarbon streams and elutes with the light naphtha fraction. The concentration of CS2 in the naphtha fraction varies considerably, depending upon the source of the hydrocarbon stream. Typically, along with CS2 in these hydrocarbon streams are found other sulfur species sulfur as mercaptans, sulfides, and disulfides which usually are present in higher concentrations. When such CS2 containing naphtha streams are exported as feedstock to steam crackers (primarily for ethylene production), it can result in the production of off-spec C5 derivatives. This then affects the downstream production quality of, among other things, synthetic rubber. Normally, CS2 is present at very low levels (<10 ppm). Along with CS2 in these hydrocarbon streams are other sulfur species like mercaptans, sulfides, and disulfides which usually are in higher concentrations. Many hydrocarbon streams are treated with catalysts to change their form, such as, isomerizing, reforming, hydrocracking, etc. Sulfur and oxygen containing organic compounds deactivate these catalysts. Heretofore, the oxygenates (i.e. methanol, dimethyl ether, acetone, acetaldehyde, etc) can be removed by adsorption easily with molecular sieves. The sulfur species (mercaptans, sulfides, and substituted disulfides (dimethyldisulfide, methylethyldisulfide, diethyldisulfide etc)) are removed by adsorption. However, CS2 is too light to normally be removed with conventional sulfur removing commercial adsorbents such as 13X, 4A, and 5A. Instead of removing the CS2, usually the sulfur specification can be met by removing all of the other sulfur species present, even though the CS2 passes through.
With the advent of lower sulfur specifications for fuels, even the presence of relatively low levels of CS2 cannot be ignored. Therefore, there is a need for an adsorbent that can effectively remove CS2 that is present at low levels (10-20 ppm). Normally, hydrocarbons adsorb more strongly than CS2 in conventional zeolites (4A, 5A, 13X) so an adsorbent is needed whose pore size is appropriate to allow CS2 to be adsorbed while excluding the hydrocarbons. In addition, carbon disulfide levels can also cause catalyst poisoning in other polymer plant feedstock production. As a consequence, petrochemical naphtha (PCN) users have moved to limit feed CS2 to <1-2 ppm.